In many industrial applications, it is necessary to use aqueous solutions or water for a variety of purposes such as heat transfer systems in which the water is used in heat exchangers, cooling towers, chillers, etc. The water will come in contact with metal surfaces of the system and, when being used in a recirculating system after being exposed to or saturated with air, will have a tendency to corrode the metal surfaces with which it comes in contact. In addition, when utilizing water in a recirculating system, the metal salts which are inherently present in certain types of water such as calcium, magnesium, etc. will tend to deposit out on the surface of the metal to cause a scale. The presence of this scale on the surface of the metal will inhibit the heat transfer capability of the metal and thus reduce the efficiency of the system.
It is therefore imparative that the deposition of scale and the corrosion of the metal surfaces of the heat transfer equipment be minimized. The minimization of these problems can be accomplished by the addition of corrosion inhibitors to the water. In many instances different types of metals are used in the apparatus including iron in the form of steel, aluminum, copper, etc. Copper is known as an accelerator for the corrosion of iron and therefore any corrosion inhibitor must include a copper chelating component in order to again minimize the corrosion of the metal.
Several U.S. patents disclose different components which are used to inhibit the corrosion of metals in heat transfer systems.
Other U.S. patents also disclose various corrosion inhibiting compositions. For example, U.S. Pat. No. 3,723,347 claims the use of diamine phosphonate plus zinc compounds, triazoles and hexavalent chromates as well as a blend of silicates along with the diamine phosphonates while U.S. Pat. No. 4,209,487 is similar to this patent while teaching that the use of zinc salts and chromates which were used in the former patent have been found to adversely affect the quality of water when released in natural waters. Therefore, this reference omits the use of such compounds. Another U.S. Pat. No. 3,714,066 also discloses a similar corrosion inhibiting composition which utilizes a soluble zinc salt and an ethane diphosphonate. Other references such as U.S. Pat. No. 3,992,318 claim the use of phosphonates or diphosphonates in combination with phosphates and water-soluble polymers of acrylic or methacrylic acid; U.S. Pat. No. 4,101,441 discloses the use of a corrosion inhibitor consisting of an azole, a water-soluble phosphate and a water-soluble phosphonate; U.S. Pat. No. 4,176,059 claims the use of a corrosion inhibitor comprising a water-soluble molybdate ion, a surfactant, an inorganic water-soluble polyphosphate, and an azole; U.S. Pat. No. 4,217,216 claims the use of an azole, molybdate and a phosphonate, and states that when the molybdate was omitted, the inhibition of the corrosion was decreased. However, as will hereinafter be shown, when a molybdate is used in combination with the other elements of our corrosion inhibiting composition, the opposite is true and corrosion inhibition will be increased; while U.S. Pat. No. 4,206,075 and B-336-566 both claim the use of specific phosphonates as well as zinc, chromates and thiols or azoles as corrosion inhibitors. In all of the references, the corrosion rates are relatively high when compared to the corrosion rate of the present invention, said corrosion rates ranging from 1.1 up to 3.1 MPY.
It is noted that none of the references disclose the specific combination of the four components which make up the corrosion inhibiting compositions of the present invention. It was totally unexpected that by combining these components it was possible to prepare a composition which, in addition to possessing an inhibition rate greater than 99%, will also eliminate pitting or crevice corrosion of the metal surface.